1. Field of the Invention
The present invention concerns a method for the determinating mechanical properties of elongated textile test material, in which the test material is clamped and stretched between two clamping devices and the occurring tensile force is measured.
2. Description of Related Art
Known tensile test equipment for the testing of yarns, twines, narrow tapes and other flexible materials are equipped with two clamping devices. These are mounted vertically one over the other, with a selectable spacing between them. The upper clamping device is connected to a load cell via a rigid connection. The lower clamping device can be moved vertically by means of a drive. A test piece clamped in the clamping devices is subjected to a tensile load by the downward movement of the lower clamping device. The tensile force occurring in the test piece is transmitted to the load cell through the upper clamping device. An incremental transducer connected to the drive records as the extension of the test piece the distance traversed by the lower clamp from the commencement of the test to the breakage of the test piece.
This measuring arrangement is flawed. The coupling of the clamping device and the load cell constitutes a preload on the load cell which impairs its sensitivity. This preload increases the mass inertia and reduces the characteristic frequency of the force measuring system. This influence has a particularly interfering effect if the occurring breaking forces are small in relation to the maximum possible load of the load cell. The test appliances themselves are designed for tensile forces of 500 N to 5 KN and above. This problem is countered with interchangeable load cells and clamping devices. For an extensive range of test material, the breaking forces can extend from a few CN into the range of several KN. The provision of the necessary load cells and clamping devices involves a high level of cost.
A further flaw in the measuring system of this equipment is the fact that the change in the distance between the clamping devices is used for measuring the extension. During a tensile test, the test material is clamped into the clamping devices. The tensile stress prevailing in the test material continues in the direction of pull in both clamping devices and only attains the value zero within the clamping faces. This reduction of tensile force within the clamping faces results in an extension which is not recorded with the measurement method described. The measured extension value is too large.
The actual extension is the change in the distance between the clamps minus twice the "clamp slippage". In the case of test materials with small extension values, this measurement error is negligible, particularly if it is relative to a long clamping length. In the case of elastic test materials with high extension and transverse contraction values, such as rubber-like materials or partially stretched synthetic yarns, or soft-annealed metal wires, this error assumes a magnitude which is no longer negligible. This measurement error is again increased considerably if the clamping length is reduced from the standard value of 500 mm to 100 mm or even 50 mm. Shortened clamping lengths are selected in order to load elastic test materials to breaking point within the greatest possible clamp distance.
The present invention is intended to eliminate the influence of the weight of the clamping device on the force measuring system and the measurement of the clamp slippage is intended to increase the accuracy of the extension measurement.
This object is achieved according to the invention in that the test material between the clamping devices is deflected by an angle and a force couple is formed as a result and in that its resultant force is used for determination of the strength properties.